In a distributed base station with remote radio, the base station is divided into two mutually independent parts: baseband unit (Base-band Unit, BBU), and remote radio unit (Remote radio unit, RRU). The RRU is placed at the access point far away from the BBU. They are connected through fibers to transmit a baseband signal in an analog or digital mode. A distributed antenna system (Distributed Antenna System, DAS) is similar to a distributed base station with remote radio. However, the distance between the BBU and the RRU is extensible to thousands of kilometers or even tens of thousands of kilometers. Moreover, the BBU may be connected to the RRU through a fiber directly or through an optical transport network such as passive optical network (Passive Optical Network, PON) or wavelength division multiplexing (Wavelength Division Multiplexing, WDM). Further, interference between cells is reduced and the system capacity is improved through a multi-cell joint processing mode such as network multiple-input multiple-output (Multiple-Input Multiple-Output, MIMO) system or multi-cell joint scheduling.
Currently, a Cloud Radio Access Network (C-RAN) system based on a cloud computing technology is attracting attention of the industry. The C-RAN is a larger radio access system formed through a cloud computing technology on the basis of the DAS technology. Compared with the DAS, the C-RAN connects the BBUs of multiple base stations through fibers or an optical transport network, and uses the cloud computing technology to virtualize the processing resources of all BBUs into a uniform resource pool. In this way, the system can implement statistical multiplexing of signal processing resources, which reduces the system cost significantly. In addition, like the DAS, the C-RAN can enhance the system capacity by means such as multi-cell joint processing.
As shown in FIG. 1, FIG. 1 is a schematic diagram of C-RAN system architecture in the prior art. The C-RAN system includes multiple C-RAN nodes. The multiple C-RAN nodes are interconnected through high-capacity fibers or optical transport networks. Each C-RAN node is connected with the RRU in a small-cell cluster (Small-Cell Cluster) in a star or ring mode through a fiber directly or through an optical transport network. Each C-RAN node is primarily responsible for radio access of users (RS) in its own small-cell cluster, including physical-layer signal processing, media access control (Media Access Control, MAC), and radio resource management (Radio Resource Management, RRM). When the processing load of a C-RAN node is light, namely, when the user traffic volume in its own small-cell cluster is not great, the C-RAN node can assist in handling radio access for a part of users in a small-cell cluster of other C-RAN nodes. When the user traffic volume in a small-cell cluster of a C-RAN node is too great so that the corresponding C-RAN node can hardly handle radio access of all users in its small-cell cluster efficiently in time, the baseband radio signals of a part of cells may be routed, through a high-capacity fiber or optical transport network connected to all C-RAN nodes, onto the light-loaded C-RAN node with low user traffic volume in the small-cell cluster.
Referring to FIG. 2, FIG. 2 is a schematic structural diagram of a C-RAN node in the prior art. FIG. 2 shows only main functional modules of a C-RAN node. A practical C-RAN node further includes other functional modules such as timing unit, control unit, and interface unit. As shown in FIG. 2, a C-RAN node may include multiple BBUs. Each BBU is responsible for handling physical-layer signals of some users, possibly including MAC/RRM processing. The C-RAN node further includes a forwarding unit. The forwarding unit is connected with all RRUs, and is also connected with other C-RAN nodes, and is configured to forward the baseband signal of the RRU connected to the C-RAN, and the baseband signal from other C-RAN nodes, onto each BBU for processing. The RRU is primarily responsible for implementing functions of a transceiver (TRX) module. That is, in the downlink direction, the RRU converts a downlink baseband signal into a radio frequency signal, amplifies the power of the signal, and then transmits the signal through an antenna; in the uplink direction, the RRU receives the uplink radio frequency signal from the antenna, amplifies the signal, and converts the signal into a baseband signal.
In the practice, the inventor finds that: With emergence of the third generation (3G) and fourth generation (4G) mobile communication technologies such as long term evolution (Long Term Evolution, LTE), the radio spectrum is wider and wider (20 MHz-100 MHz). Meanwhile, the multi-antenna technologies such as MIMO are applied massively, which makes the transmission bandwidth wider and wider between the C-RAN node and the RRU. Therefore, it is very important to reduce the required signal transmission bandwidth between the C-RAN node and the RRU.